Composite containing reinforcing fibers comprising carbon

ABSTRACT

Composite which contains reinforcing fibers comprising carbon and whose matrix comprises silicon carbide, silicon and copper, with the mass fraction of copper in the composite being up to 55%, processes for producing it, in particular by liquid infiltration of C/C intermediate bodies with melts comprising Si and/or Cu and Si, and also its use as friction lining in a friction pairing with ceramic brake discs or clutch discs comprising C/SiC

FIELD OF THE INVENTION

[0001] The invention relates to a composite containing reinforcingfibers comprising carbon. This composite comprises, in its matrix,phases of silicon carbide, silicon and copper and is suitable asfriction partner for fiber-reinforced ceramic counterbodies, inparticular bodies made of the material system C/SiC. The inventionfurther relates to a process for producing this composite, in particularby liquid infiltration of C/C intermediate bodies by Si- andCu-containing melts, and to the use of this composite as frictionmaterial in friction pairings with ceramic brake discs or clutch discsmade of C/SiC.

BACKGROUND OF THE INVENTION

[0002] In the search for suitable friction materials forhigh-performance brake systems using ceramic friction bodies, theconventional organically bound friction materials reach their limitsbecause of the high temperatures and high wear rates which occur. Thisbecomes particularly apparent in the case of friction pairings in whichC/SiC or C/C-SiC is used as brake disc material. These materials areceramics formed essentially of SiC and secondary phases comprising Siand C, which is reinforced with carbon fibers, for example as describedin DEA 197 10 105. During braking, temperatures around and above 1000°C. occur at the friction surface, and organically bound brake liningsconsequently decompose.

[0003] In EP-A 1 079 137, a sintered metal material is proposed forbrake linings which, in combination with C/C-SiC brake discs, leads toan increased operating life and improved frictional behavior andconsists of a sintered copper material having a mass fraction of morethan 60% of copper. However, inter alia, the high-temperature strengthrequired for ceramic friction partners is restricted by the high coppercontent and the binder phase.

[0004] In DE-A 197 27 586, it is proposed to use a combination of aC/SiC brake disc and a corresponding C/SiC brake lining. The C/Cintermediate body for the brake disc has a density which is lower thanthat of the surface regions of the C/C intermediate body for the brakelining. This leads to a C/SiC having a relatively low strength beingformed after the liquid silicization of the C/C intermediate body forthe lining. However, the overall frictional and wear behavior, theperformance when wet and in particular the comfort characteristics,including constant coefficients of friction and low noise, are not yetsatisfactory.

[0005] In view of this prior art, it is an object of the invention toprovide a friction material which withstands the high temperaturesduring braking without suffering damage and displays improved comfortcharacteristics in combination with ceramic counterbodies. Inparticular, the friction material should be matched to the combinationwith C/SiC brake discs.

SUMMARY OF THE INVENTION

[0006] According to the invention, this object is achieved by acomposite which is reinforced with carbon fibers and whose matrixcomprises silicon carbide, silicon and optionally carbon, together withcopper. In this composite, copper is present in elemental form or asalloy predominantly as precipitates or as Cu-rich phase within thematrix of the C/SiC composite.

[0007] The invention accordingly provides a composite suitable asfriction material which contains reinforcing fibers comprising carbonand whose matrix comprises silicon carbide, silicon and copper, with themass fraction of copper in the composite being up to 55%.

[0008] The matrix of the composite can further comprise carbon.

[0009] Compared to the prior art, this friction material for the firsttime combines the good high-temperature and friction properties of C/SiCand the ability of Cu to modify the coefficient of friction while at thesame time obtaining a homogeneous material which is stable at hightemperatures.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] The mass fraction of copper in the composite is preferably in therange from 10 to 50% and particularly preferably in the range from 15 to45%.

[0011] The copper in the matrix is preferably present in isolatedcopper-containing regions, with the copper-containing phases not forminga continuous phase but the individual copper-containing regions insteadbeing surrounded completely by other phases, in particular phasescomprising silicon carbide. Such noncontiguous copper-containing regionsare referred to as “discrete phases”.

[0012] The volume fraction of discrete, copper-containing phases in thecomposite is preferably up to 25%, based on the volume of the composite.

[0013] The copper-containing phases preferably comprise not only copperbut also further metals which form mixed phases with copper. Preferenceis given to the metals zinc, tin, lead and aluminum which may be presentindividually or in admixture. The mass fraction of these metals, basedon the sum of the masses of copper and these metals, is up to 25%.

[0014] Copper is already known as additive in brake linings to alter thecoefficient of friction or the thermal conductivity, in particular incombination with brake discs made of gray cast iron or steel. However,these lining compositions are organically bound linings or sinteredmetal linings which are decomposed at temperatures around or above about1000° C., with the copper even melting and destroying the lining.

[0015] Although the friction lining of the invention also containscopper in elemental form or in the form of its alloys whose meltingpoints can be reached or exceeded at the use temperatures, surprisinglyno destruction of the composite due to melting takes place. This isprobably attributable, inter alia, to the microstructure of the materialin which Cu is embedded in the SiC-containing matrix.

[0016] Apart from the copper-containing phases and silicon carbide,other carbides can also be present in the matrix. Particular preferenceis given to carbides of boron, titanium, zirconium, hafnium, vanadium,chromium, molybdenum, tungsten, iron, cobalt and nickel. Furthermore,the matrix can further comprise silicides of the abovementionedelements. It is also possible for these elements to be present asmetallic phases in the matrix. The mass fraction of these elements andtheir carbides and silicides is preferably up to 20% of the mass of thematrix.

[0017] Preference is also given to a composite further comprisingpreformed particulate additives which are added in the production of theC/C composite and form separate phases in the composite. Preferredparticulate additives are silicon carbide, silicon nitride, titaniumcarbide, aluminum oxide (corundum), zirconium dioxide, silicon dioxideand zirconium silicate. These particulate additives are preferably usedin such amounts that their mass fraction is up to 15% of the mass of thematrix.

[0018] The invention further provides a process for producing suchfriction materials, which comprises the steps

[0019] production of a porous carbon/carbon composite (C/C or CFRC(=carbon fiber reinforced carbon) body, i.e. a body whose matrixconsists essentially of carbon and which is reinforced with carbonfibers and is porous,

[0020] melt infiltration of the porous C/C body with a metal meltcomprising silicon and copper,

[0021] reaction of at least part of the carbon of the C/C body with thesilicon of the melt to form silicon carbide.

[0022] The metal melt used comprises not only silicon and copper butgenerally also small amounts, typically from 0.1 to 5%, of furthermetals.

[0023] The C/C body is produced by known methods and preferably containscoated short carbon fibers or fiber bundles as reinforcing fibers. TheC/C body has to have open pores which allow access of the metal meltinto the interior of the body. According to the invention, the porositymeasured as the volume of pores as a proportion of the total volume ofthe body is at least 10% and preferably at least 18%. The preferreddensity of the C/C body is not more than 1.6 g/cm³, preferably in therange from about 1 to 1.5 g/cm³.

[0024] In the process of the invention, it is preferred that the C/Cbody itself contains further particulate additives for the frictionmaterial, in particular grains of hard material or sintered metalparticles. As hard material additives, preference is given to usingcarbides or oxides in mass fractions of up to about 15%, preferably upto 15% and particularly preferably from 1 to 13%, with their meltingpoint advantageously being above the melting point of silicon. Typicalrepresentatives of such hard material additives are silicon carbide,silicon nitride, titanium carbide, aluminum oxide (corundum), zirconiumdioxide, silicon dioxide and zirconium silicate.

[0025] In a further advantageous embodiment of the process of theinvention, metallic copper or its alloys with mass fractions of up to25%, preferably from 1 to 20% and particularly preferably from 2 to 15%,of other metals is introduced into the C/C body itself, since thisallows the proportion of copper to be introduced via the melt to bereduced. Here, the copper or its alloys is preferably introduced in theform of powder or turnings. In the extreme case, it is even possible forthe introduction of copper from the outside via the melt to be omittedentirely, so that only a silicon melt is used for infiltration.

[0026] The introduction of copper or copper-containing alloys ispreferably carried out by introducing copper powder or copper turningsor powder or turnings of copper-containing alloys into a shapedthermoset body reinforced with carbon fibers (CFRP body) which iscarbonized in a known manner on heating in the absence of oxygen totemperatures in the range from about 800 to about 950° C. The shapedthermoset body preferably comprises polymers giving a high carbon yield,e.g. phenolic resins, furan resins, epoxy resins or polyimides, ascarbonizable material. Any hard material additives or sintered metalparticles used are preferably also introduced into the CFRP body.

[0027] Melt infiltration is usually carried out at temperatures of atleast 1450° C. under atmospheric pressure or reduced pressure, underprotective gas or in vacuo. As metal melts, use is made of Cu/Si meltsin which further elements, in particular carbide-forming elements, canbe present, in which case these are then coinfiltrated. Preference isgiven to using boron and, as further metals, Ti, V, Cr, Mo, W, Fe, Coand Ni or mixtures of two or more of these elements in mass fractions ofup to about 20%, particularly preferably from 1 to 20% and in particularfrom 2 to 15%, in the melt. This group of metals has, in particular, thetask of favorably influencing the reactivity of the silicon toward thecarbon of the C/C body and the oxidation behavior of the frictionmaterial at high use temperatures. Under the conditions of theinfiltration and reaction, carbides and silicides of these elements areformed, in addition to unreacted residues of the additives.

[0028] The metal melt is preferably supplied via porous wicks ofcarbon-containing material and/or via beds of metal particles. A mixtureof powder or granules of the individual metal components Si and Cu istypically used for this purpose.

[0029] It is not necessary for the Si/Cu alloy corresponding to thedesired Si/Cu ratio to be produced beforehand, since the Si and Cupowders or granules combine on thermal treatment to give a joint melt.However, it can be advantageous to use previously produced, finishedalloys since melting them generally requires significantly lowertemperatures. This makes the melt infiltration process technicallysimpler. A significant advantage of the use of previously producedalloys for infiltration is that the copper melt, which generally haspoor wetting properties, is taken up into the capillaries and pores ofthe C/C body without application of external pressure with the aid ofthe silicon which is the main constituent of the melt and has very goodwetting properties. This advantage becomes clear in comparison with, forexample, the process described in DE-A 37 31 540. Here, a frictionmaterial comprising C/C with Cu-filled pores is produced. Cu is forcedunder pressure into the continuous pores. Closed pores in the C/C bodycannot be filled in this way.

[0030] In the filling of the pores, too, the process of the inventiondisplays significant advantages, since the closed pores can also befilled by melt in this way. This is achieved by part of the carbon ofthe C/C body being consumed by the reaction with silicon, which canresult in closed pore channels being opened. Copper can then also enterthe previously closed pores together with the silicon. The liquidinfiltration process using copper-containing silicon melts leads to atypical microstructure of Cu phases or precipitates surrounded by SiCand/or Si. In particular, Cu phases or precipitates which no longertouch the surface of the composite but are enclosed by the surroundingC/SiC material are formed. In a further advantageous embodiment of theinvention, a Cu alloy in place of Cu is used in admixture with siliconfor the infiltration. As alloy constituents, preference is given to themetals zinc, tin, lead and aluminum which are preferably present in massfractions of up to 25%.

[0031] The volume fraction of Cu phases or Cu alloy phases in thecomposite is preferably up to 25%, particularly preferably up to 22% andin particular from 5 to 20%. This volume fraction refers to those phaseswhich consist essentially of Cu (in an amount of at least 30% of theirmass).

[0032] In the infiltration of the C/C body with silicon and the reactionof the carbon with the silicon to form silicon carbide, the carbonfibers are also attacked, at least on their surface. In this way, thefibers are coated with a layer of silicon carbide. Preference is givento the carbon fibers not having any direct contact with acopper-containing phase, but the carbon fibers and the copper-containingphases instead being separated from one another by, at least, a thinsilicon carbide layer formed in this way.

[0033] The composite of the invention is particularly useful as frictionmaterial for friction pairings with counterbodies in which at least thefriction surface is made of hard ceramic material. It is preferably usedin friction pairings with counterbodies of composite ceramic reinforcedwith carbon fibers and having an SiC-containing matrix, particularlypreferably C/SiC ceramic. The composite of the invention is particularlyadvantageous in brake linings for ceramic brake discs which have a massfraction of silicon carbide of at least 60% in the zone facing the brakelining.

1. A composite which contains reinforcing fibers comprising carbon andwhose matrix comprises silicon carbide, silicon and copper and in whichthe mass fraction of copper is up to 55%, wherein the reinforcing fibersare coated.
 2. The composite as claimed in claim 1, wherein the copperin the matrix is present in isolated copper-containing regions which arecompletely surrounded by other phases, in particular phases comprisingsilicon carbide.
 3. The composite as claimed in claim 2, wherein thevolume fraction of the copper-containing regions in the composite is upto 25%, based on the volume of the composite.
 4. The composite asclaimed in claim 2, wherein the copper-containing phases comprise coppertogether with further metals which form mixed phases with copper and areselected from among zinc, tin, lead and aluminum, with these being ableto be present individually or in admixture.
 5. The composite as claimedin claim 4, wherein the mass fraction of these metals is up to 25%,based on the sum of the masses of copper and these metals.
 6. Thecomposite as claimed in claim 1, wherein the matrix further comprisescarbides and/or suicides of the elements boron, titanium, zirconium,hafnium, vanadium, chromium, molybdenum, tungsten, iron, cobalt andnickel and/or these elements themselves.
 7. The composite as claimed inclaim 6, wherein the sum of the mass fractions of these elements, theircarbides and suicides is up to 20% of the mass of the matrix.
 8. Thecomposite as claimed in claim 1 which further comprises preformedparticulate additives which form separate phases in the composite. 9.The composite as claimed in claim 8, wherein the particulate additivesare selected from among silicon carbide, silicon nitride, titaniumcarbide, aluminum oxide, zirconium dioxide, silicon dioxide andzirconium silicate.
 10. The composite as claimed in claim 8, wherein themass fraction of the particulate additives is up to 15% of the mass ofthe matrix.
 11. A process for producing composites as claimed in claim1, which comprises the steps production of a porous carbon/carboncomposite containing coated carbon fibers, melt infiltration of theporous C/C body with a metal melt comprising silicon and copper,reaction of at least part of the carbon of the C/C body with the siliconof the melt to form silicon carbide.
 12. The process as claimed in claim11, wherein the metal melt used comprises silicon and copper togetherwith a mass fraction of from 0.1 to 5% of further metals.
 13. Theprocess as claimed in claim 11, wherein the C/C body prior toinfiltration has a pore volume of at least 10% of the total volume ofthe body and a density of not more than 1.6 g/cm³.
 14. The process asclaimed in claim 11, wherein the C/C body prior to infiltration containsfurther particulate additives selected from among grains of hardmaterial and sintered metal particles.
 15. The process as claimed inclaim 14, wherein carbides or oxides having melting points above themelting point of silicon are used in mass fractions of up to about 15%as hard material additives.
 16. The process as claimed in claim 11,wherein metallic copper or its alloys with mass fractions of up to 25%of other metals is/are introduced into the C/C body itself.
 17. Theprocess as claimed in claim 16, wherein the copper or its alloys areintroduced in the form of powder or turnings.
 18. A process forproducing composites as claimed in claim 1, which comprises the stepsproduction of a porous carbon/carbon composite containing coated carbonfibers and copper, melt infiltration of the porous C/C body with asilicon melt, reaction of at least part of the carbon of the C/C bodywith the silicon of the melt to form silicon carbide.
 19. The process asclaimed in claim 11, wherein the metal melt used is a Cu/Si melt whichfurther comprises additional carbide-forming elements selected fromamong boron, titanium, vanadium, chromium, molybdenum, tungsten, iron,cobalt and nickel and mixtures of two or more of these elements in massfractions of up to about 20% of the melt.
 20. The process as claimed inclaim 11, wherein the metal melt is supplied via porous wicks made ofcarbon-containing material and/or via beds of metal particles.
 21. Theprocess as claimed in claim 11, wherein previously produced, finishedalloys of Cu and Si are used for infiltration.
 22. The process asclaimed in claim 11, wherein a Cu alloy is used in admixture withsilicon for the infiltration, with the metals zinc, tin, lead andaluminum being used as alloy constituents in mass fractions of up to 25%of the mass of the alloy.
 23. A method of use of a composite as claimedin claim 1 for producing friction materials, comprising producing aporous carbon/carbon composite body, infiltrating the said body with ametal melt comprising silicon and copper, and forming silicon carbide inthe said body.
 24. A method of use of a composite as claimed in claim 1in friction pairings with counterbodies in which at least the frictionsurface is made of hard ceramic material, comprising combining thecomposite of claim 1 in the form of a friction pad with a frictionsurface.
 25. The method of use as claimed in claim 23 wherein thefriction surface is a composite ceramic reinforced with carbon fibersand having an SiC-containing matrix.
 26. The method of use as claimed inclaim 25 wherein the friction surface is a ceramic brake disc which hasa mass fraction of silicon carbide of at least 60% in the zone facingthe friction pad in the form of a brake lining.